1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include one or more base stations or access points for providing wireless connectivity to mobile units in a geographic area (or cell) associated with each base station or access point. Mobile units and base stations communicate by transmitting modulated radiofrequency signals over a wireless communication link, or air interface. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like.
Mobile units can initiate communication with the base station by transmitting a message on one or more of the random access channels (RACHs). Uplink random access channels are non-synchronized and therefore may be transmitted at any time relative to the synchronized downlink timing by any mobile unit within the coverage area of the base station. The receiver in the base station must therefore continuously monitor the random access channels and search the signals received on the random access channels for predetermined sequences of symbols (sometimes referred to as the RACH preamble) in random access channels transmitted by mobile units. To make the search process feasible, the format of the random access channels must be standardized. For example, conventional random access channels in the Universal Mobile Telecommunication Services (UMTS) Long Term Evolution (LTE) system are transmitted in a subframe during a transmission time interval (TTI) of 1 ms in 1.08 MHz bandwidth.
The reception times for uplink random access of signals transmitted by mobile units near the center of the cell and mobile units near the edge of the cell can be offset by as much as the round-trip delay corresponding to the cell radius. The offset arises because the non-synchronized random access uplink signals for a particular subframe are transmitted relative to the arrival times of the corresponding synchronized downlink subframe. A mobile unit at the center of the cell receives the synchronized downlink subframe earlier than mobile unit at the edge of the cell (by approximately the one way delay corresponding to the cell radius) and uplink signals transmitted from central mobile units arrive at the base station earlier than uplink signals transmitted from edge mobile units (by approximately the one-way delay corresponding to the cell radius). Inter-symbol interference between random access channels associated with different subframes occurs if random access signals associated with one subframe overlap with a subsequent subframe and therefore interfere with random access signals associated with the subsequent subframe. Inter-symbol interference can be reduced by including a guard time in each random access channel subframe during which no uplink signal is transmitted to reduce or prevent inter-symbol interference. For example, the random access channel subframe can be divided into a 0.8 ms preamble and a 102.6 μs cyclic prefix that includes a copy of a portion of the sequence of symbols in the preamble. The remaining 97.4 μs in the transmission time interval is reserved as a guard time
The coverage area of a base station is related to the duration of the cyclic prefix and the guard time. For example, the conventional guard time of approximately 0.1 ms corresponds to a round-trip delay for a signal that travels approximately 15 kilometers. Thus, a random access channel format that includes approximately 0.1 ms for the guard time is appropriate for reducing or preventing inter-symbol interference for coverage areas or cell sizes having a radius of up to approximately 15 kilometers. Similarly, the duration of the cyclic prefix is related to the size of the coverage area. For example, a cyclic prefix of approximately 0.1 ms is suitable for coverage areas having radii of up to approximately 15 kilometers. Although a range of 15 km may be considered sufficient for conventional wireless communication systems, the base station range of proposed wireless communications systems, such as the UMTS LTE, is expected to increase to at least 100 km in scenarios with good radio propagation conditions such as coverage in coastal areas.
Proposals to extend the range of the random access channel supported by base stations include increasing the transmission time interval to 2 ms. For example, one proposal includes changing the structure of random access channels. In this proposal, the extended transmission time interval includes a 0.8 ms RACH preamble. The length of the cyclic prefix (CP) also increases in proportion to the desired coverage area. For example, every 0.1 ms of additional cyclic prefix length will account for additional 15 km coverage. The guard time also increases at the same rate as the cyclic prefix length. Thus, with the 0.8 ms RACH preamble, the time available for guard time and cyclic prefix is 2 ms−0.8 ms=1.2 ms. This RACH range extension proposal attempts to reduce the receiver complexity of the RACH preamble detection. However, the guard time and the cyclic prefix are considered pure overhead because no new information can be transmitted during these intervals. Increasing the guard time or the cyclic prefix length much beyond the current value of 0.1 ms is therefore not considered a desirable way to extend the range of cells because of the high resource cost.
In other proposals, two partitions between cyclic prefix and guard time (or guard period) can be envisioned: In one case, the 1.2 ms portion of the subframe that is not allocated to the preamble could be evenly allocated to the cyclic prefix and the guard time so that the RACH coverage is extended to 90 km. Alternatively, the 1.2 ms portion of the subframe that is not allocated to the preamble could be unevenly distributed between the cyclic prefix length and the guard time. The uneven distribution of the allocated time to the cyclic prefix and the guard time could extend the coverage to the 100 km if the cyclic prefix length is equal to or greater than 0.667 ms. However, inter-symbol interference may occur when the cyclic prefix and guard time allocations are uneven in cases where the preamble is transmitted by a mobile unit near the cell edge. Moreover, the signal strength received from mobile units in the edge of an extended cell, e.g., mobile units that are as much as 90 or 100 km from the base station, may be very low, which may reduce the likelihood of detecting the preamble of the random access channel.